Refrigerating apparatuses are recently demanded to have a high operating efficiency. A compressor driven by a linear motor, having a substantially simple mechanical structure, has been utilized widely for improving the operating efficiency since the compressor is expected to significantly decrease loss in its sliding movement.
FIG. 10 is a cross sectional view of a conventional linear motor disclosed in Japanese Patent Laid-open No. 9-172764. A second silicon steel sheet layer 3 of a hollow cylindrical shape having a coil 2 is provided by a gap on the outer side of a first silicon steel sheet layer 1 of a hollow cylindrical shape. Plural magnets 5 are bonded and fitted into grooves provided in the outer side of a nonmagnetic shell 4 of a hollow cylindrical shape. The shell is located between the first silicon steel sheet layer 1 and the second silicon steel sheet layer 2 and is joined to a piston (not shown), thus forming a magnetic assembly 6. A movable section of the motor including the magnet assembly 6 is arranged for reciprocate movement along the axial direction D1 of the first silicon steel layer 1 and the second silicon steel layer 2.
The magnets 5 are made of rare-metal ferromagnetic material for generating a high intensity of magnetic field for allowing the motor to have an operating efficiency. The magnets are magnetized vertical to the direction of the reciprocate movement.
An operation of the conventional linear motor will be explained.
The coil 2, upon being energized with a current, generates a magnetic flux loop through the first silicon steel sheet layer 1, a gap, the magnet 5, a gap, the second silicon steel sheet layer 3, a gap, the magnet 5, a gap, and the first silicon steel sheet layer 1, hence forming a magnetic circuit. The magnetic flux causes the magnets 5 to be attracted by magnetic poles developed on the second silicon steel sheet layer 3. Then, as the current to the coil 2 is alternated, the magnetic assembly 6 carries out a reciprocate motion between the first silicon steel sheet layer 1 and the second silicon steel sheet layer 3 along the direction D1 in FIG. 10.
The conventional linear motor includes the magnetic assembly 6 to reciprocate between the first silicon steel sheet layer 1 and the second silicon steel sheet layer 3, and thus needs the gaps among the magnetic assembly 6, the first silicon steel sheet layer 1, and the second silicon steel sheet layer 3. This arrangement has the magnetic flux loop developed on the first silicon steel sheet layer 1 and the second silicon steel sheet layer 3 for driving the magnetic assembly 6 flow across the gaps.
The gaps of the magnetic assembly 6 from the first silicon steel sheet layer 1 and the second silicon steel sheet layer 3 are designed to be a desired distance for preventing them from any direct contact. The gaps, however, act as magnetic resistances, thus decreasing the intensity of the magnetic flux in proportion to the distance. This increases a current to the coil 2 in order to offset the decreasing of the intensity of the magnetic flux, which results from the gaps, and makes the motor need a sufficient power for driving the magnetic assembly 6. As the result, the conventional linear motor consumes more energy, and thus, it is difficult to increase the operating efficiency of the motor.
The conventional motor requires greater sizes of magnets 5 in order to generate a necessary power for driving the magnetic assembly 6. The greater size of the magnets 5 made of rare-metal material, being expensive, increases the overall cost of the motor.
The gap between the magnetic assembly 6 and the first silicon steel sheet layer 1 and the gap between the magnetic assembly 6 and the second silicon steel sheet layer 3 are preferably identical in their distances. If the gaps are not equal in the distances, the magnetic attraction between the magnets 5 and the first silicon steel sheet layer 1 may be different from that between the magnets 5 and the second silicon steel sheet layer 3. This creates a pinching stress perpendicular to the movement of the magnetic assembly 6. The stress causes a supporting mechanism, such as a bearing, to produce a loss in sliding movement and an abnormal worn-out, thus shortening its life time.
For avoiding the above problem, the distances of the gaps may be increased to relatively reduce the difference between them. This, however, requires a further increase in a current input and increase the size of the magnets 5. Thus, improving the dimensional accuracy of a driving system including the magnet shell is commonly considered. For improving the dimensional accuracy, the magnet shell 4 as a moving component has to have an increased thickness, thus increasing the overall weight of the driving system. This makes a force for driving the magnet assembly 6 increase, and thus, increases the current input to the coil 2. Moreover, as the dimensional accuracy of the driving system is increased, its overall production cost is raised.